Magnetic resonance imaging at high-field, e.g., 3-4.7 T, and ultra-high field, 7 T and above, strengths is a natural and sustained evolution for the use of magnetic resonance imaging as a diagnostic tool, since higher field strengths lead to higher signal levels and improved image quality. However, physical limitations stand in the way of fully efficient high-field and ultra-high field imaging protocols.
T2-weighted (T2W) imaging is one of the most useful and fundamental types of imaging methods for magnetic resonance imaging clinical routines. Performing T2W imaging of the brain, however is a challenge at ultra-high field strengths of 7 T or higher. Specifically, at high field, T2W imaging is limited by transmit field inhomogeneity and increased radiofrequency power deposition.
A popular method for T2W imaging is turbo spin echo, which offers fast acquisitions using multiple refocusing radiofrequency pulses. Multi-echo spin echo sequences are used extensively to generate T2W images at 1.5 T and 3 T, but do not perform optimally at ultra-high field strengths.
First, multi-spin echo sequences prescribe a large flip angle, which is extremely sensitive to the B1 inhomogeneity present at a field strength of 7 T. The inhomogeneous transmit field leads to signal loss and variable T2 contrast across the imaged volume.
Second, refocusing radiofrequency pulses uses large amounts of power to achieve slice selectivity and the large flip angles associated with multi-spin echo sequences. This leads to high power deposition and limited scan efficiency. Specifically, the large radiofrequency power deposited by the refocusing pulses, leads to significant increases in required scan time in order to remain below specific absorption rate (SAR) limitations at 3 T and above.
Diffusion-weighted (DW) imaging is another important technique in magnetic resonance imaging. DW imaging is used to characterize lesion malignancy and to visualize neuronal fiber orientation and integrity. Standard DW imaging sequences use the spin echo as a basic building block, which leads to similar issues as T2W imaging when employing DW imaging sequences at ultra-high field strengths.
One alternative to spin-echo-based T2W and DW imaging is to play a T2 magnetization preparation (T2-prep) followed by a fast imaging acquisition. Although utilizing T2-prep provides a significantly lower power usage, the T2-prep method has a high sensitivity to B0 and B1 inhomogeneity, which results in signal loss and banding artifacts due to incomplete refocusing or imperfect tip-down or tip-up projections. Advanced radiofrequency pulse designs have been proposed to improve robustness of the T2-prep method. However, all such designs perform T2 weighting in the whole imaged volume, resulting in inefficient acquisitions because of long repetition times required for T1 recovery.